The present invention relates to an ink jet fluid system and, more particularly, to such a system in which sub-atmospheric pressure may be maintained within the print head of an ink jet printer after termination of printing operations to prevent subsequent fluid flow through the print head orifices.
Ink jet printers, such as disclosed in U.S. Pat. No. 3,891,121, issued June 24, 1975, to Stoneburner, are known in which ink is supplied at a pressure in excess of atmospheric pressure to a manifold communicating with a series of small diameter orifices. As the ink flows through the orifices under pressure, it forms fine filaments of fluid which break up into jets of discrete drops. At the point at which the drops break away from the filaments, they pass through electrically conductive charging rings to which charging potentials are selectively applied to charge selected ones of the drops. An electrostatic deflecting field, extending across the paths of the jet drop streams, deflects the charged drops into catch trajectories. A catcher is provided to catch the charged drops such that they are prevented from striking a print receiving medium. Uncharged drops, however, pass unaffected through the deflection field and are deposited upon the print receiving medium.
In the operation of such a printer, the fluid supplied to the manifold must be maintained at a pressure in excess of atmospheric pressure in order to produce the flow of fluid through the orifices and the resulting fluid filaments. After operation of the printer, however, it is important to terminate the fluid flow through the orifices such that the print head, charge rings, and other printer elements do not become wetted with ink in such a manner as to short out these elements and inhibit subsequent printer operation. Additionally, if ink should weep through the print head orifices, this ink may become dried and block the orifices.
The approach taken by Stoneburner, as disclosed in the Stoneburner '121 patent, is to terminate the supply of ink to the print head and simultaneously to replace this ink with a supply of a flushing fluid, such as a cleaning solvent liquid. Thereafter, the supply of flushing fluid is terminated and a line is opened from the manifold to a waste sump operating at a pressure substantially below atmospheric pressure. This sudden reduction of pressure in the manifold is sufficient to terminate the jets of flushing fluid, without producing masses of fluid at the orifices or the formation of erratic drops of fluid. Thereafter, air is supplied to the manifold to dry the manifold. It may be preferable in some applications to maintain ink within the manifold after shut down of the printer. The ink would have to be maintained at a pressure less than atmospheric pressure, however, to prevent weeping of the ink through the orifices. If a sub-atmospheric fluid pressure were maintained within the print head manifold by a pump system, however, such a pressure would not be maintained during a power failure.
Where ink is used to flush a print head by supplying the ink to a print head inlet and simultaneously removing it from a print head outlet, it is also important that air is not introduced into the ink as it is removed from the print head if the ink is to be returned to the system fluid supply.
U.S. Pat. No. 4,152,710, issued May 1, 1979, to Metsuba et al., discloses a fluid supply system for an ink jet printer in which a cross valve connects the printer nozzle to a tank at shut down. The tank has an opening in the upper portion thereof, for maintaining the internal pressure of the tank at atmospheric pressure and an inlet in the bottom of the tank which is connected by the cross valve to the nozzle. Ink from the tank is returned to the fluid supply system through an outlet opening in the side wall of the tank. The outlet opening is maintained at a height substantially identical with that of the nozzle such that the pressure of the ink at the nozzle at shut down is substantially equal to atmospheric pressure. A float within the tank closes the vent opening if the level of the fluid in the tank is raised by an abnormally large fluid flow to the tank during actuation of the cross valve. This system makes no provision for preventing air from entering the fluid supply system through the vent opening of the tank and, additionally, maintains the fluid pressure at the nozzle during shut down at substantially atmospheric pressure, with the result that some weeping of fluid out of the nozzle opening may occur.
A number of ink jet systems have incorporated a valve arrangement or other device in the return line from a printer catcher or gutter such that caught drops may be returned to the printer fluid supply system and resupplied to the print head without the introduction of air into the fluid supply system. One such arrangement is shown in U.S. Pat. No. 3,761,953, issued Sept. 25, 1973, to Helgeson et al. Helgeson incorporates a float valve in the return line from the printer catcher. A float within the valve is raised and lowered by the fluid within the valve so as to open and close a valve seat in the bottom of the valve.
U.S. Pat. No. 3,929,071, issued Dec. 30, 1975, to Cialone et al. discloses a printer in which the catcher return line includes a J-shaped tube in which a residual quantity of ink is held, thus sealing the return line and preventing air from being drawn into the line. Finally, Japanese Pat. No. 54-13337, dated Jan. 31, 1979, discloses a gutter having a float valve arrangement mounted therein which opens the gutter fluid outlet after a sufficient quantity of ink has been accumulated by the gutter.
Accordingly, it is seen that there is a need for an ink jet fluid system which provides a sub-atmospheric pressure within the print head after shut down, but which precludes air from being drawn into the return line from the print head to the fluid supply system.